Vacuum lift foam filled casting system

Abstract

A lost foam casting system (2) is provided with vacuum lift of molten metal (16) to an evaporative foam pattern assembly (8) surrounded by unbonded particulate media such as sand (6) in a flask (4). A gas permeable member (34, 70) formed of randomly oriented ceramic fibers is provided between the sand and a vertical fill passage (20) to apply vacuum from the sand to the fill passage such that molten metal is vacuum lifted through the fill passage to the foam pattern assembly. The foam material is vaporized by the heat of the molten metal as the metal advances upward and is replaced by the metal in the shape of the pattern assembly.

Description

BACKGROUND AND SUMMARY

The invention relates to a lost foam casting system, and more particularly to an improved method and apparatus for introducing molten metal to the pattern assembly.

Lost foam casting is a known technique. A pattern assembly formed of evaporative foam material, for example, gasifiable or liquifiable expanded polystyrene, or expanded polymethylmethacrylate (PMMA), either of which may ba coated with a thin ceramic coating, is placed in a flask and surrounded by unbounded particulate media, most commonly sand, though other particles can be used such as zirconia, metal spheres, etc. Upon application of molten metal to the pattern assembly, the foam material vaporizes and is replaced by the metal in the shape of the pattern assembly. The vaporized foam material escapes into the interstices in the sand.

The pattern assembly typically includes one or more workpieces of given respective patterns for yielding cast metal parts, and a sprue connected by one or more in-gates to the workpieces for communicating the molten metal through the sprue to the in-gates and then to the workpieces. After cooling, the cast metal sprue and the in-gates are broken away from the work-pieces and discarded or recycled. The cast metal workpieces are then trimmed to yield the final product.

The present invention arose out of efforts to provide improved cast aluminum alloy cylinder heads and engine blocks for internal combustion engines in marine applications, and cast steel two-cycle outboard crankshafts, though the invention is not limited thereto.

In the present invention, molten metal is applied to the foam pattern assembly by vacuum lift. Vacuum is applied through the sand, and a gas-permeable member or path is provided communicating between the sand and a vertical fill passage such that molten metal is vacuum lifted to the foam pattern assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional view of a lost foam casting system in accordance with the invention;

FIG. 2 is a sectional view of a portion of FIG. 1;

FIG. 3 is a view like FIG. 2 and shows an alternate embodiment; and

FIG. 4 is a view like FIG. 2 and shows another alternate embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a lost foam casting system 2 in accordance with the invention. A flask 4, such as a cylindrical open topped barrel, is provided for holding unbonded particulate media 6, preferably sand, though other particles can be used, as noted above. Pattern assembly 8 is formed of evaporative foam material, for example gasifiable or liquifiable expanded polystyrene, or expanded polymethylmethacrylate (PMMA), either of which may be coated with a highly permeable thin ceramic coating. Pattern assembly 8 is gas-nonpermeable. Flask 4 has a bottom wall 10 with a central opening 12 therein through which a gas-nonpermeable fill tube 14 extends. A source of molten metal 16, such as provided by a furnace or reservoir 18 is disposed below flask 4. Fill tube 14 extends downwardly from the bottom of the flask and defines a vertical fill passage 20 communicating pattern assembly 8 with the source of molten metal. Flask 4 has peripheral flanges such as 22 and 24 for supporting the flask from eye hooks such as 26 and 28 and chains or cables such as 30 and 32 which in turn are supported from a hoist (not shown).

Flask 4 is initially in a raised position above molten metal source 18. Fill tube 14 is placed in the flask from above and inserted downwardly into and through opening 12 in the bottom of the flask and supported from bottom wall 10, to be described. An annular frustconical gas-permeable throat 34 is then inserted partially into fill tube 14 from above. Pattern assembly 8 has a lower stem 36 with a bottom end 38 inserted partially into throat 34 until collar 40 on stem 36 is stopped against the top 42 of the throat. Sand is then poured into flask 4 surrounding throat 34 and pattern assembly 8. The sand is then compacted by vibration. The flask is then closed by top cover 44 which has a central fitting 46 therethrough beneath which is preferably a sand filter or screen 48. Flask 4 is then lowered such that the lower portion of downwardly depending fill tube 14 enters the molten metal 16 in source 18. A source of vacuum 50 is then connected to fitting 46 to apply vacuum to flask 4 and sand 6 therein, as monitored by vacuum gauge 52.

Gas-permeable throat 34 communicates between sand 6 and fill passage 20, which passage extends upwardly through throat 34 to pattern assembly 8. The vacuum is thus applied from sand 6 through gas-permeable throat 34 to fill passage 20 such that molten metal 16 from source 18 is vacuum lifted through fill passage 20. The foam material of pattern assembly 8 vaporizes and is replaced by the metal in the shape of pattern assembly 8. The vaporized foam material escapes into the interstices in sand 6. The heat radiation from the molten metal entry causes the foam to gasify, and the molten metal may or may not touch the foam prior to such gasification. Flask 4 is then raised in a timed manner such that when the casting inlet gates freeze the vacuum is released and the remaining molten metal in the gating system drains back into molten metal source 18. After sand dumping and cooling, the cast metal parts are broken way and trimmed, etc.

Fill tube 14 is a tubular member having an upper flange 54 engaging the underside of a refractory gas-nonpermeable annular disc 56. Flange 54 is in opening 12 and has a smaller diameter than opening 12. Disc 56 has an outer diameter larger than the diameter of opening 12 and engages the top side of bottom wall 10 of the flask. Disc 56 has a central opening 58 therethrough further defining vertical fill passage 20. A top annular ring 60 engages the top of disc 56, and bolts such as 62 and 64 extend through disc 56 bolting ring 60 and flange 54 to the disc. Ring 60 has a central opening 66 through which throat 34 extends downwardly. The inner diameter of throat 34 is substantially constant and of a width about the same as stem 36 and slightly smaller than the diameter of the depending tubular member of filler tube 14. The outer diameter of throat 34 frustoconically tapers downwardly and engages the inner diameter of disc 56. Throat 34 extends downwardly through opening 66 and through opening 58, and the bottom end 68 of throat 34 is at the level of flange 54, though other configurations are of course possible. Throat 34 spaces the pattern assembly above the filler tube. The gas-permeable path through throat 34 extends generally horizontally below the pattern assembly and above the fill tube, transversely through the generally vertical side walls of throat 34. The vacuum communication path through gas-permeable member 34 is generally transverse to the vertical lift path of the molten metal through fill passage 20.

FIG. 3 shows an alternate embodiment of the gas-permeable member and fill tube arrangement and uses like reference numerals from the above figures where appropriate to facilitate clarity. Throat 34 is replaced by a gas-permeable gasket 70 on flange 54 of fill tube 14. Stem 36 of pattern assembly 8 rests on gasket 70. Flange 54 rests on the top side of bottom wall 10 of the flask and the fill tube depends therefrom through central opening 12a. Gas-permeable gasket 70 communicates vacuum from sand 6 to fill passage 20, as in FIG. 1.

The gas permeable members 34 and 70, as shown in FIGS. 2 and 3, respectively, consist of a vacuum formed fibrous ceramic material composed of a multiplicity of chopped, randomly oriented ceramic fibers. The members 34,70 are inert to the molten metal, have a low thermal conductivity and a specific gravity generally in the range of 0.5 to 0.6. The ceramic fibers have a melting point above the melting point of the metal being cast, and can take the form of aluminum oxide, magnesium oxide, zirconium oxide, silicon carbide, or the like. A vacuum is drawn in the flask, and as the evaporable foam pattern is impermeable to gas flow, gas will be drawn from the fill tube through the wall of the gas permeable member and into the particulate material or sand in the flask to thereby lower the pressure in the fill tube and initiate the cavity filling of molten metal. As the molten metal is drawn upwardly through the fill tube, it flows through the central opening or passage in the member 34,70 and into contact with the pattern assembly 36. The heat of the molten metal will vaporize the pattern assembly with the vapor being entrapped in the interstices of the sand and the molten metal occupying the void created by vaporization of the foam pattern assembly to provide a cast metal part having a configuration corresponding to that of the pattern assembly.

FIG. 4 shows another alternative gas-permeable means and fill tube arrangement and uses like reference numerals from the above figures where appropriate to facilitate clarity. The gas-permeable means is provided by one or more vacuum passages such as 72 and 74 formed through pattern assembly 8 and/or fill tube 14. FIG. 4 shows one such vacuum passage 72 formed by one or more exposed grooves along the underside of the pattern assembly, as along the bottom 38a of stem 36a of the foam pattern assembly. FIG. 4 also shows a passage 74 formed by one or more exposed grooves along the topside of the fill tube, as along the topside 76 of flange 54 of fill tube 14. In the embodiment of FIG. 4, the pattern assembly at its bottom edge 38a engages the fill tube at its top surface 76, and the vacuum passages 72 and 74 are at the interface 38a-76 of the pattern assembly and the fill tube. The vacuum passages extend generally radially outwardly and transversely from passage 20. Vacuum passages 72 and 74 communicate between sand 6 and fill passage 20 to apply vacuum from sand 6 through such vacuum passages 72 and 74 to passage 20 to lift the molten metal through such fill passage. In the embodiment in FIG. 4, the bottom stem 36a of the foam pattern assembly has a lower widened skirt portion 78 to engage the top of the fill tube, rather than nesting therein.

Pattern assembly 8 includes in this example one or more workpieces such as 118 and 120 of given respective patterns for yielding cast metal parts. The assembly further includes a central vertical sprue 122 connected by one or more in-gates such as 123-125 and 126-128 to respective workpieces for communicating molten metal through sprue 122 to the in-gates 123-128 and then to the workpieces 118 and 120. In the particularly disclosed embodiment, the sprue is a hollow square member open at the top and has six in-gates on a side, up to a total of twenty-four if all four sides are used. In principle the sprue could be much simpler, for example just a solid cylinder. The six in-gates pictured in the example shown in FIG. 1 are arranged in two vertical columns of three in-gates each, each of which sets of three in-gates is connected to a workpiece, such as cylinder head, as for example in-gates 123-125 are connected to cylinder head 118. There are thus two cylinder heads per side of sprue 122, providing a total of up to eight cylinder heads. Each pair of cylinder heads on a side are interconnected by foam spacing members (not shown).

Sprue 122 is a one-piece molded member. In-gates 123-125 are formed with cylinder head 118 and then glued to the flat side of the sprue. Alternatively, the in-gates may be molded with the sprue and then glued to the respective cylinder heads, or further alternatively the in-gates may be formed as separate members and glued to the sprue and glued to the respective cylinder head. During vaporization of the foam material of the pattern assembly, the glue also melts and is vaporized and escapes into the interstices in the sand. It is desirable to reduce the number of glue joints to reduce the amount of glue which must be vaporized because excessive amounts of same will cause carbon related imperfections in the final cast metal part.

Sprue 122 has four vertical feed passages interconnected by the four support walls forming the square and providing a relatively rigid structure. The feed passages are at the corners and pass substantially more molten metal than the support walls therebetween. The in-gates are fed with molten metal from respective feed passages. The support walls are thin flat members connected edge to edge to define a square exterior perimeter surrounding a hollow interior. The exterior perimeter has the noted four corners at the respective intersections of the flat walls. The feed passages are at the respective corners and have a substantially larger cross sectional thickness than the flat support walls therebetween and bulge inwardly from the corners such that the exterior of the sprue is flat from corner to corner to facilitate workpiece and in-gate placement. The placement of the in-gates is relatively noncritical because of the flat exterior surface of the sprue. Furthermore, the relatively rigid structure of the sprue prevents distortion during the fluidization cycle and hence maintains close mechanical tolerances, which is extremely important for cylinder heads.

One or more of the vertical thin flat support walls such as front wall 140 has a pair of apertures 148 and 150 for passing sand therethrough to fill the hollow interior of the sprue, in addition to sand filling the hollow interior of the sprue through its open top, to provide high surface area sand contact to volume ratio to maximize escape of vapor and to promote rapid cooling.

Sprue 122 which, as noted above, could be as simple as a solid expanded PMMA cylinder, has the noted lower stem portion 36 for receiving molten metal through fill passage 20 from source 18, and has a distribution portion provided by a plurality of fingers or spokes such as 159 and 160 extending radially outwardly from stem 36 and then vertically upwardly to the noted feed passages at the corners of the sprue. The sprue is open, as shown at 161, between such spokes or fingers. Stem 36 may be a solid foam member, but preferably is at least partially hollowed out to reduce bulk density and buoyancy during the liquification process and to reduce wasted metal.

It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

Claims

1. A lost foam casting apparatus comprising, a flask for holding unbonded particulate material, a source of molten metal external of said flask, a pattern assembly composed of an evaporable foam material disposed in said flask and surrounded by said particulate material, fill means having a fill passage providing communication between said flask and said source of molten metal, means for applying a vacuum to said flask and to said particulate material, and an annular gas permeable member disposed in said flask and having a central opening providing communication between said pattern assembly material and said fill passage, at least a portion of the periphery of said member being exposed to said particulate material, said member being composed of randomly oriented ceramic fibers permeable to the flow of gas and impermeable to the flow of molten metal, application of a vacuum to said flask drawing gas from said fill passage through said member and into said particulate material, said vacuum being of sufficient magnitude to cause molten metal to be drawn through said fill passage from said source of molten metal and through the opening in said member and into contact with said foam material to vaporize said foam material with the vapor being entrapped within the interstices in said particular material and said molten metal filling the void created by vaporization of said foam material.

2. The apparatus of claim 1, wherein said ceramic fibers are composed of metal oxides.

3. The apparatus of claim 1, wherein a first end of said central opening communicates with said fill passage and a second end of said central opening communicates with said pattern assembly.

4. The apparatus of claim 3, wherein said pattern assembly includes a section disposed within said second end of said central opening.

5. The apparatus of claim 1, wherein said flask has an aperture in the bottom wall thereof, the upper end of said fill tube having an annular flange resting on the upper surface of said bottom wall, said gas permeable member resting on the upper surface of said flange, said foam material being supported on the upper surface of said member and extending across the opening in said member.

6. A lost foam casting apparatus comprising, a flask for holding unbonded particulate material, a source of molten metal external of said flask, a pattern assembly composed of an evaporable foam material disposed in said flask and surrounded by said particulate material, fill means having a fill passage providing communication between said flask and said source of molten metal, means for applying a vacuum to said flask and to said particulate material, an annular gas permeable member disposed within the flask and having a central passage, one end of said central passage communicating with said fill passage and the opposite end of said central passage being closed off by said pattern assembly, at least a portion of the outer periphery of said member being in direct contact with said particulate material, said member being composed of randomly oriented ceramic fibers constructed and arranged to permit the flow of gas through the wall of said member and to prevent the flow of molten metal through said wall, said ceramic material having a melting point above the melting point of said metals, application of a vacuum to said flask drawing gas from said fill passage through said member and into said particulate material, said vacuum being of sufficient magnitude to cause molten metal to be drawn through said fill passage from said source of molten metal and through the opening in said member and into contact with said foam material to vaporize said foam material with the vapor being entrapped within the interstices in said particular material and said molten metal filling the void created by vaporization of said foam material.